opennet: increase maxpeers to 140

Document the considerations for choosing max peers and increase
the max peers to 140 and the panic limit to 150 to take the
current gap between slow and fast connections into
account. Currently about 20% of our users max out the 100 peers
limit.

--HG--
extra : amend_source : ed3b947966c6da40791630e2fc144e62934e5767
opennet manager: clearer comment

Replaced the imagined distance level by the simple hop. The imagined
distance level referred to a simplistic network model with several
stages of link lengths instead of the more organic link building
freenet uses. It did not help understanding.

--HG--
extra : source : 8bc2ae5b4a00dc3bd98efaef28bdf45b9c5e7de9